This invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps, and in particular to such ballasts having a minimum number of active components.
Most magnetically coupled self-oscillating inverters are manufactured in large quantities for sale in a highly competitive market. Half-bridge inverters generally have a lower cost because of a reduced parts count. Such inverters may be classified into two groups: those using a current transformer having a saturable core, generally together with power BJT's (bipolar junction transistors); and those using a current transformer having a linear core, generally together with MOSFETs (metal oxide semiconductor field effect transistors). As those of ordinary skill will recognize, in this context a linear core is one in which operation is over a region having a curved B-H characteristic, rather than a sharp B-H characteristic; that is, at all times the flux level is such that a significant increase in magnetizing current will be accompanied by a significant increase in flux level.
U.S. Pat. No. 5,349,270 discloses a MOSFET half-bridge ballast which has auxiliary windings coupled to a series resonance inductor which is connected between the load and the inverter output at the node between the two switching transistors. The load, a single 20 w fluorescent lamp, is in series with a 220 nf capacitor, and 10 and 15 nf capacitors are in a starting circuit in parallel with the lamp. These capacitors form a resonant circuit with the inductor. As a result, the signals provided by the auxiliary windings are proportional to the inverter output current. Each of the auxiliary windings is connected to a low-pass phase shifting circuit whose output provides a control signal to the gate electrode of the respective MOSFET. It will be clear to those of ordinary skill in the lamp ballast art that, if this ballast were applied to a multiple lamp fixture, removal of one lamp while the ballast was operating would cause a great change in the output voltage applied to the remaining lamp(s). The simple low-pass filter structure is not able to accommodate the proper frequency changes when the lamp number is changed.